1. Field of the Invention
This invention relates to a process for the selective production of para-xylene by catalytic methylation of toluene in the presence of an ammonium hydrogen phosphate treated crystalline aluminosilicate zeolite catalyst.
2. Description of the Prior Art
Alkylation of aromatic hydrocarbons utilizing crystalline aluminosilicate catalysts has heretofore been described. U.S. Pat. No. 2,904,607 to Mattox refers to alkylation of aromatic hydrocarbons with an olefin in the presence of a crystalline metallic aluminosilicate having uniform pore openings of about 6 to 15 Angstrom units. U.S. Pat. No. 3,251,897 to Wise describes alkylation of aromatic hydrocarbons in the presence of X- or Y-type crystalline aluminosilicate zeolites, specifically such type zeolites wherein the cation is rare earth and/or hydrogen. U.S. Pat. No. 3,751,504 to Keown et al. and U.S. Pat. No. 3,751,506 to Burress describe vapor phase alkylation of aromatic hydrocarbons with olefins, e.g., benzene with ethylene, in the presence of a ZSM-5 type zeolite catalyst.
The alkylation of toluene with methanol in the presence of a cation exchanged zeolite Y has been described by Yashima et al. in the Journal of Catalysis 16, 273-280 (1970). These workers reported selective production of paraxylene over the approximate temperature range of 200.degree. C. to 275.degree. C., with the maximum yield of para-xylene in the mixture of xylenes, i.e., about 50 percent of the xylene product mixture, being observed at 225.degree. C. Higher temperatures were reported to result in an increase in the yield of meta-xylene and a decrease in production of para- and ortho-xylenes.
U.S. Pat. No. 3,965,207 describes methylation of toluene to selectively yield para-xylene at a temperature between about 500.degree. C. and about 750.degree. C. in the presence of a ZSM-5 type zeolite catalyst. U.S. Pat. No. 3,965,208 describes the methylation of toluene, under conditions such that the formation of meta-xylene is suppressed and the formation of ortho- and para-xylene is enhanced, carried out in the presence of a catalyst comprising a crystalline aluminosilicate zeolite of the ZSM-5 type which had been modified by the addition thereof of a small amount of a Group VA element.
While the above noted prior art is considered of interest in connection with the subject matter of the present invention, the methylation process described herein utilizing a catalyst of a crystalline aluminosilicate zeolite having a silica/alumina ratio of at least about 12, a constraint index within the approximate range of 1 to 12 and which has undergone prior modification by treatment with an ammonium hydrogen phosphate to deposit at least about 0.5 weight percent of phosphorus thereon and thereby achieve unexpectedly high selective production of para-xylene has not, insofar as is known, been heretofore described.
Of the xylene isomers, e.g., ortho-, meta and paraxylene, the latter is of particular value being useful in the manufacture of terephthalic acid which is an intermediate in the manufacture of synthetic fibers such as "Dacron". Mixtures of xylene isomers either alone or in further admixture with ethylbenzene, generally containing a concentration of about 24 weight percent para-xylene in the equilibrium mixture, have been previously separated by expensive superfraction and multistage refrigeration steps. Such process, as will be realized, has involved high operation costs and has a limited yield.